As is known, an optical data medium, such as a CD, DVD, CD-R or similar item, can be sensed optically and the data of the recorded image points can be evaluated in order to detect any potential flaws. In general, light is applied to the data side and is reflected from the reflecting layer of the optical data medium and shines on an optical receiver, for example, a CCD-camera. The light is often shaped as line light which extends across a radius of the data side of the optical data medium. The optical data medium is rotated about its rotational axis perpendicular to its flat sides so that, to check one entire side, at least one complete rotation of the data medium is required.
Only one data side is used on a CD so that this kind of sensing need be performed only from one side. However, in the case of a DVD, both opposing, flat sides are designed as data sides. In this case, checking of both sides is necessary. Basically, a DVD is configured such that it has two outer, transparent, plastic layers coated with a reflective layer and an adhesive layer provided as an intermediate layer which joins the two outer plastic layers together. Thus, a data medium is created which can be read on both sides.
In addition to checking the finished products for potential flaws, it is also necessary to classify the flaws, on the one hand, to handle certain flaws with more strict inspection criteria, and on the other hand, to allow a specific intervention in the on-going production process. This prevents additional flaws from occurring during production. The essential factor in this case, among others, is to recognize potential flaws in the intermediate layer, since these flaws will have an effect on both data sides.
The flaws in the intermediate layer are detectable from both sides. However, each optical receiver sees only the side of the intermediate layer which it faces. Thus, as a rule, a flaw in the intermediate layer cannot be classified as such solely due to its pattern.